The present invention relates to a system for coding and/or decoding signal including a luminance signal and carrier chrominance signals quadrature-modulated by two chrominance signals.
The NTSC color television signal, for example, is a composite signal including a luminance signal (Y signal), and carrier chrominance signals quadrature-modulated by two chrominance signals (I and Q signals) and frequency-multiplexed at frequencies lying in the higher frequency region of the luminance signal. Prior art techniques have provided the separate-coding system for efficiently coding the composite color television signal, in which the composite signal is demodulated into baseband signals (e.g., Y signal, and I and Q signals) which are then converted into band-compressed signals and coded under the optimized coding conditions for the individual signals. The separate-coding system is efficient because of band-compression coding optimum for the individual baseband signals.
In the separate-coding system, however, the carrier chrominance signals must be demodulated and remodulated, which necessitates intricate circuit design. Furthermore, picture impairment is inevitable during modulation and demodulation of signals. Still further, the coding system needs numbers of coders, in addition to a multiplexer for multiplexing the coded signals, complicating the system construction.
As a solution to the problem, prior art techniques have resorted to the direct-coding system such as the higher-order DPCM (differential pulse code modulation) using a higher-order predictive function adapted to the composite color television signal, and the orthogonal conversion coding based on Hadamard conversion, in which the composite color television signal is coded directly without being demodulated into a baseband signal. This coding system obviates the need for demodulation and remodulation of the chrominance signal as in the separate-coding system and therefore permits system construction to be simplified.
In the direct-coding system, however, the sampling frequency must be more than twice the signal frequency band and hence has had to be as high as 9 to 11 MHz, simply because this system is to directly encode the composite color television signal. In practice, coding efficiency is lower in the direct-coding system than in the separate coding system.